EP 1 117 731 B1 describes polycarbonate foils which are almost optically isotropic, and also a chill-roll process for their production. The foils are composed of linear or branched polycarbonate whose average molecular weight Mw is from 10,000 to 40,000. Inventive foils can by way of example have shrinkage of <1%/<1% (190° C./30 min machine direction (MD)/transverse direction (TD)). There is preferably no region of the surface of the inventive foil that deflects linearly polarized light by more than 2 angular minutes. By way of example the path difference can be 31 nm, the birefringence delta n 4×10−4. By way of example, the tensile strain at break measured to ISO 527-3 MD/TD in the tensile test is 39 and, respectively, 35%. To avoid die lines or extrusion streaks it is recommended that the inner surface of the extrusion die used has a roughness depth to DIN 4768 of from 0.025 to 0.002 in the die lip region. This can be achieved via chroming and polishing.
The advantageous properties of the material are in particular said to be achievable by keeping the temperature of the polymer melt at least 30° C. above the glass transition temperature of the plastic after discharge from the extrusion die until contact is made with the chill roll. To this end, the path from discharge from the extrusion die to the point of application to the chill roll can be selected to be relatively short, e.g. in the region of by way of example from 30 to 150 mm. The emerging melt film can moreover be thermally screened by means of a housing or additionally heated. The process is suitable for a wide variety of plastics, in particular for norbornene plastics.
U.S. Pat. No. 5,476,134 describes extruders coated with chromium nitrides for production of parts composed of aluminium-iron alloys. These are particularly suitable for reducing the level of erosion of the extruder parts when producing aluminium alloys with very small proportions of iron below 0.3%.
EP 351 886 (Bayer AG) describes a casting or extrusion process for production of optically uniaxial, birefringent polycarbonate foils, which are heated by means of a quartz radiant heater. The result is a reduction in the undesired birefringence to values that are no longer problematic.
JP 8336883 describes a process for extrusion of foils composed of thermoplastic, where the radius of the edges of the discharge orifice of the extrusion die used is 30 μm or less, with non-uniformity of not more than +/−20%. The die geometry described in particular counters formation of undesired deposits.
JP 6335949 describes a process for extrusion of foils composed of thermoplastic, preferably by way of example polyether sulphones, polyphenylene oxides or polyether ketones, where the radius of the edges of the discharge orifice of the extrusion die used is 30 μm or less, with non-uniformity of not more than +/−20%. The extrusion die can be composed of steel or ceramic. Coatings with chromium, nickel, titanium, copper, zinc, etc. can counteract any possible interactions of the die material with the extruded plastics materials. The die geometry described in particular counteracts formation of die lines.
JP 2002-028941 describes a process for production of foils composed of polyvinyl alcohol, and these can be used as polarization films. Films with reduced thickness variations can be obtained if the radius of the edges of the discharge orifice of the extrusion die used is 200 μm or less.
JP 2003-267758 describes a process for coating of optical fibres with a plastics sheathing. Here, the radius of the edges of the discharge orifice of the extrusion die used is from 10 to 30 μm. The die geometry described in particular counteracts formation of plastics deposit in the region of the edge of the die, so that the process can then be operated over a prolonged production period without disruption. Another advantage is that the plastics coating has lower thickness variations.
EP 0 547 562 A1 describes a process for coating of a sheet-like web which runs around a roll, with a coating solution. Here, the coating solution is applied from a slot die radially, i.e. at an angle of 90° with respect to the roll surface, to the web to be coated. A multilayer web is produced by virtue of the conveying motion of the roll. The steep angle of application achieves a choke effect in relation to the amount of coating solution applied. If the distance between the die and the roll or the corresponding gap between die and roll deviates excessively from the intended situation the result can be impairment of the surface quality of the multilayer web, e.g. lines and streaking. There is therefore a distance measurement system installed on the right- and left-hand side of the coating die, with a control circuit, in order to exert an advantageous influence on the reproducibility and precision of the process. If the distance between the discharge orifice of the coating die and the roll deviates from the intended value, the position of the coating die with respect to the roll is corrected correspondingly by means of an adjuster.
EP 1 202 261 A1 describes optical storage media. The vertical birefringence of commercially available, extruded polycarbonate foils produced by the chill-roll process is compared inter alia with that of commercially available, cast polycarbonate foils. The extruded foils have vertical birefringence values of 0.00029, while the cast foils have values of 0.00066.